Test the effects of mutagens in enhancing frequency of recombination (and mutation) within the H-2 complex of mice. Screen the progeny of parents heterozygous for two recombinant H-2 haplotypes for enhanced recombination frequencies. Initiate a new series of crosses for recombinant screening spanning only the K--G interval of the H-2 complex. Develop and apply MLR and Ia serotyping methods for detection of I region mutants and recombinants. Establish a congenic inbred strain homozygous for each new variant H-2 haplotype detected for supply to other laboratories.